7. PROJECT SUMMARY AND RELEVANCE: While Na+ intake exacerbates hypertension, blood pressure also rises with increased Cl- intake, independently of Na+, making Cl- intake and renal Cl- absorption critical in blood pressure regulation. Aldosterone administration in vivo increases NaCl uptake by the cortical collecting duct (CCD) primarily through principal cell-mediated Na+ absorption and intercalated cell-mediated Cl- absorption, which occurs in exchange for HCO3-, largely through the Cl-/HCO3- exchanger, pendrin. How Cl- contributes to hypertension and, in particular, how aldosterone stimulates IC Cl-/HCO3- exchange is poorly understood. Aldosterone?s signaling mechanism in principal cells is well established. 11 ?HSD2 metabolizes cellular glucocorticoids enabling aldosterone binding to the MR (Nr3c2), thereby stimulating ENaC. However, published and preliminary data show that the signaling mechanism(s) of aldosterone and the MR are very different in ICs. First, 11 ?HSD2 is not expressed in ICs. Second, MR inhibitors reduce IC transporter abundance even in the known absence of aldosterone. Third, published and preliminary data show angiotensin II and a high NaCl diet can activate the MR independently of aldosterone, likely through a mechanism involving Rac1, a GTPase in the Rho family, that regulates cytoskeletal reorganization and gene expression. We observed that Rac1 gene ablation or Rac1 inhibition reduces pendrin abundance in angiotensin II-treated mice. We observed that increased NaCl intake reduces Cl- absorption and Cl- transporter abundance in CCDs from untreated mice, but increases the apical plasma membrane abundance of intercalated cell Cl- transporters in angiotensin II-treated mice. We therefore hypothesize that IC MR activation occurs through or independently of aldosterone. We also hypothesize that angiotensin II-induced IC MR activation occurs through Rac1 and the MR, which act a molecular switch, by which the effect of NaCl intake IC Cl- transporter abundance and function (IC Cl-/HCO3- exchange) changes from inhibitory to stimulatory. To study IC transporter regulation by the IC MR, we developed mice lacking the mineralocorticoid receptor and Rac1 in intercalated cells (IC MR KO and IC Rac1 KO mice). Determining how aldosterone and angiotensin II target intercalated cells may provide additional targets for hypertension. Proposal Aims are to determine: 1) if the mineralocorticoid receptor modulates intercalated cell transporter abundance and function, blood pressure and salt balance, 2) if the mineralocorticoid receptor stimulates intercalated cell function independently of aldosterone or aldosterone binding and 3) If angiotensin II acts through the MR and Rac1 to modulate the effect of NaCl intake on IC transporter abundance and function. We will examine the effect of K+, aldosterone, angiotensin II and the MR on intercalated cell transporter abundance and function in vivo and in vitro with quantitative real time PCR, immunohistochemistry, immunogold cytochemistry, renal tubules perfused in vitro and in whole animal studies.